1. Field of the Invention
The invention relates generally to a gas cleaning process and apparatus, and more particularly relates to a process and apparatus that utilize electrostatically charged fine screens for promoting agglomeration of small particles in a gas stream into larger clusters and removing particles entrained in the gas stream.
2. Description of the Related Art
Industrial electrostatic precipitators (ESPs) are used in coal-fired power plants, the cement industry, mineral ore processing and many other industries to remove particulate matter from a gas stream. ESPs are particularly well suited for high efficiency removal of very fine particles from a gas stream. Specially designed ESP's have attained particle collection efficiencies as high as 99%.
Conventional ESPs typically remove 90-99% of the flyash and dust in the flue gas. Fuel switching and sulfur control systems upstream of the ESPs modify flyash properties and reduce precipitator collection efficiency. In addition, conventional ESPs are inefficient in capturing sub-micron sized particles. Toxic trace metals and their compounds, as well as heavy organics, tend to concentrate on fine particulates in the range of 0.1-2.0 microns. Faced with increasingly stringent environmental requirements, utilities that produce such gases are looking for alternative solutions, low cost retrofits or complete replacement of their precipitators in order to capture all of these materials.
One way to overcome these problems is to replace the existing under-performing ESPs with “baghouse” filters. Although such filters operate with very high collection efficiencies (greater than 99.9%), independent of flyash properties, they possess very low filtration velocities, they are large and, therefore, require significant space, they are costly to build, and are therefore unattractive for retrofitting of existing precipitators. Reducing the size of such filters by increasing filtration velocity results in substantial pressure drops, which are known to be disadvantageous. There is a potential for blinding of the bags, and even fire. Because of the small bag spacing, much of the dust removed from one row of bags is re-collected on the adjacent rows of bags.
Most recently, efforts have been made to install baghouse filters as the polishing units installed after conventional ESPs, such as in “Compact Hybrid Particulate Collector” (known as COHPAC) disclosed in U.S. Pat. No. 5,158,580. Additionally, it is known that electrical enhancement of filtration results in a reduction of pressure drop. Hence some baghouses have electrostatically enhanced fabric filtration section, such as in U.S. Pat. No. 6,152,988.
It is also possible to use perforated structures and grids set in the direction perpendicular to that of gas flow. The prior art includes U.S. Pat. Nos. 1,381,660; 1,479,271; 3,616,606; 3,668,836; 5,593,476; 5,695,549 and 6,585,803. In U.S. Pat. Nos. 1,381,660 and 1,479,271, grounded screen collection electrodes are preceded by vertical, spaced-apart wire discharge electrodes. However, because of large apertures or spacing, none of these discharge electrodes can produce substantially uniform corona discharge to charge the particles passing through, although this charging is a prerequisite for a successful capture of particles, especially very fine ones.
U.S. Pat. No. 3,616,606 discloses a two-stage precipitator. The first stage is similar to that in conventional precipitators and is used to pre-charge the particles and only partially collect them. The second stage consists of a plurality of electrically charged corona-free perforated structures with “one or more apertures” of unspecified size, set perpendicular to the gas flow and used to “slow down” the particulate matter and to collect earlier charged particulate by charging the perforated electrodes with different charged formations. However, such electrodes are not meant and cannot be used for corona production.
U.S. Pat. No. 3,668,836 discloses the use of vertical, electrically charged, ionizing rods/wires that are spaced out upstream of a plurality of grounded, perforated collection electrode plates with 0.5-inch or larger holes set perpendicular to the gas flow direction.
U.S. Pat. No. 5,695,549 discloses a so-called “agglomerator” that is installed before a conventional precipitator in order to agglomerate small particles for easier capture by the precipitator. The particulate matter passes at very high speeds (50 ft/s) through a series of parallel and oppositely charged pairs of discharge and collection screen electrodes with openings as large as 0.25 to 1.0 inch. The discharge electrodes have pointed, protruding elements for forming a substantially uniform corona discharge. Unlike previously mentioned disclosures in which the grid spacing and/or aperture size is selected to maximize the collection of charged particles, the agglomerates in this system are intended to re-entrain into the gas stream to be subsequently removed by conventional precipitator. It is to be expected that collection plates or grids with such large openings in both U.S. Pat. No. 3,668,836 and U.S. Pat. No. 5,695,549 would have problems in efficiently collecting and/or agglomerating fine particles.
U.S. Pat. No. 5,593,476 discloses an apparatus that utilizes a combination of large-opening grids, and a fibrous filter that is polarized by a high potential difference between electrodes. U.S. Pat. No. 6,585,803 discloses utilization of a sintered, stainless steel fibrous filter in a so-called point-to-plane electrostatic precipitator operating at low filter face velocities.
There is a need for a high efficiency collection device that is easily retrofitted into existing space, causes agglomeration of particles into larger clusters and efficiently removes all large and very fine particles from the gas stream.